The present invention is directed to a bidirectional communications system formed with a monomode light waveguide, wherein it is possible to provide a light source or light sources at only one end of the light waveguide link and to provide a reflecting means with an optical modulator at the other end of the light waveguide link.
Bidirectional communications systems formed with a monomode light waveguide, which has one end connected to a light source or a plurality of light sources and the other end connected to a reflecting means which is actuated by an optical modulator, are disclosed, for example, in U.S. Pat. No. 4,195,269, whose disclosure is incorporated by reference; U.S. Pat. No. 4,775,971, which was the basis for WO-A-No. 87/06084, and whose disclosure is incorporated by reference. In addition to the above two examples, such a system is disclosed in German OS No. 27 08 606 and an article by Wheeler et al, entitled "Two-Way Transmission Using Electro-Optical Modulator", Electronics Letters, 24 Apr. 1986, Vol. 22, No. 9, pp. 479-481 and an article by Duthie et al, entitled "Bidirectional Fibre-Optic Link Using Reflective Modulation", Electronics Letters, 8 May 1986, Vol. 22, No. 10, pp. 517-518.
An optical (intensity) modulator can be fashioned with a controllable optical directional coupler, as illustrated in FIG. 3 of the above-mentioned U.S. Pat. No. 4,775,971, and as disclosed in FIGS. 8 and 9 of an article by F. Auracher et al entitled "Entwicklungstendenzen der Integrierten Optik", telecom report, Vol. 10, No. 2, 1987, pp. 90-98.
Such an optical directional coupler comprises two identical optical strip waveguides, which are narrow, thin strips produced by a diffusion of, for example, titanium into a substrate of lithium niobate. These strips have a higher optical refractive index than the refractive index of the substrate. These two optical strip waveguides are conducted in close proximity to one another, approximately a distance of 5 .mu.m over a defined length so that optical fields of waves guided in the strip waveguide overlap and thus periodically repeated with what is referred to as the coupling length, light energy is coupled from, respectively, one strip waveguide into the other strip waveguide. Electrodes are provided between and next to the strip waveguides and external electrical fields generated between these electrodes lead to changes in the refractive index due to what is called an electro-optical effect and, thus, change the propagation speed and the amount of scatter with the result that a corresponding change in the coupling of the light energy between the two waveguides will occur.
For a bidirectional light waveguide communications system comprising a light source or light sources at only one end of the light waveguide link, the object of the invention is to, then, disclose a way to achieve an especially expedient design of a reflection transmission and reception means at the other end of the waveguide link.
In this context for a bidirectional light waveguide communications system comprising a light source or light sources, preferably formed by a laser or plurality of laser at only one end of the light waveguide, a reflection transmission means has already been disclosed that is formed by a halved, controllable optical direction coupler. The coupler has one of its input/output connected to the light waveguide and the two strip waveguides are terminated with a partially reflecting mirror. The coupler has control electrodes which are charged with a transmission signal, and an opto-electric transducer charged with the reception of the light signal by both strip waveguides can be produced following the partially reflecting mirror. This arrangement is disclosed in copending U.S. patent application Ser. No. 226,882, filed Aug. 1, 1988, which claims priority from German Patent Application No. 37 25 479.0.